1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same, and in particular to a liquid crystal display device having improved display characteristics including an improved viewing angle performance and a method for producing such a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices (hereinafter, referred to as "LCD devices") are used in, for example, planar display apparatuses for personal computers and the like, liquid crystal TVs, and portable display apparatuses. Most of the LCD devices currently used are TN (twisted nematic) type LCD devices (hereinafter, referred to as "TN LCD devices"). A TN LCD device includes a liquid crystal layer sandwiched between a pair of substrates located opposite to each other. The liquid crystal layer includes liquid crystal molecules therein. The orientation direction of the liquid crystal molecules is changed by an electric field and thus the birefringence of the liquid crystal layer is changed, thereby performing display.
FIG. 1 is a partial cross sectional view of a liquid crystal panel of a conventional TN LCD device. The liquid crystal panel is an element which performs actual display in an LCD device. As is shown in FIG. 1, the liquid crystal panel includes a pair of substrates 131 and 132 located opposite to each other, and a liquid crystal layer 133 having liquid crystal molecules 133a disposed therebetween. The liquid crystal layer 133 includes liquid crystal molecules 133a therein. The substrate 131 includes a base 131a, an electrode 131b and an alignment layer 131c laminated in this order. The substrate 132 includes a base 132a, an electrode 132b and an alignment layer 132c laminated in this order. Initially, the liquid crystal molecules 133a are tilted at a pretilt angle .delta. relative to the substrates 131 and 132, and the orientation direction of the liquid crystal molecules 133a is twisted at approximately 90.degree. from the substrate 132 to the substrate 131 (twist angle .theta.t=90.degree.). The alignment layers 131c and 132c are provided in order to put the liquid crystal molecules in such an initial orientation direction. For the alignment layers, polyimide treated by rubbing is widely used.
When a voltage is applied to each of the electrodes 131b and 132b, an electric field is applied in a direction perpendicular to the substrates 131 and 132. The liquid crystal molecules 133a are erected by the dielectric anisotropy thereof to be parallel to the direction of the electric field. Thus, the birefringence of the liquid crystal layer 133 is changed. If the liquid crystal molecules 133a are perpendicular to the direction of the electric field (pretilt angle=0.degree.), the liquid crystal molecules 133a are erected in different directions. Therefore, the liquid crystal layer 133 is divided into a plurality of domains. The liquid crystal molecules 133a included in the same domain are erected in the same direction, and the direction of erection is different domain by domain. As a result, an interface between two adjacent domains is recognized as a disclination line, which scatters light. The state in which such a disclination line is generated is referred to as "reverse tilt". In the normally white mode (hereinafter, referred to as the "NW mode"), where the light transmittance is maximum when no voltage is applied, the disclination line reduces the contrast of displayed images. In order to prevent generation of the disclination line, the liquid crystal molecules 133a are tilted at the pretilt angle as is shown in FIG. 1.
FIG. 2 is a diagram showing initial orientation directions of the liquid crystal molecules 133a in the liquid crystal panel shown in FIG. 1 seen from top of the substrate 132. In FIG. 2, vector a represents the rubbing direction of the alignment layer 132c, and vector b represents the rubbing direction of the alignment layer 131c. The liquid crystal molecules 133a in the vicinity of the alignment layers 131c and 132c are oriented along the respective rubbing directions (vectors a and b in FIG. 2) with the pretilt angle .delta.. The rubbing directions a and b make an angle of 90.degree., which provides a twist angle .theta.t=90.degree.. Since the liquid crystal molecules 133a have a twist angle of 90.degree. as is described above, the liquid crystal molecules 133a in a central area in the thickness direction of the liquid crystal layer are oriented in the direction indicated by vector c in FIG. 2 with the pretilt angle .delta. relative to the substrates 131 and 132. The liquid crystal molecules are directed toward the substrate 132 (top substrate) in the direction indicated by vector c.
Vector c represents a direction in which the liquid crystal molecules 133a in the central area are oriented (referred to as the "reference orientation direction"). The reference orientation direction (vector c in FIG. 2) is a two-dimensional concept in a plane of the substrate. The dashed line represents a plane perpendicular to vector c and bisecting the liquid crystal panel. In this specification, a viewing angle or direction which is right with respect to the dashed line (FIG. 2) will be referred to as "positive", and a viewing angle (.theta.v in FIG. 1) or direction which is left with respect to the dashed line will be referred to as "negative". As is understood from FIG. 2, the reference orientation direction (c) bisects the twist angle .theta.t. The direction opposite to the reference orientation direction will be referred to as the "reference viewing direction v". The reference viewing direction v is included in the positive viewing directions.
Further in this specification, orientation directions of the liquid crystal molecules will be indicated using a hypothetical clock face. In detail, where the liquid crystal panel is located in the usual direction for viewers, the top part of the liquid crystal panel will be referred to as "12 o'clock", and the bottom part will be referred to as "6 o'clock". The reference orientation direction of the liquid crystal molecules will be represented in this manner. For example, the liquid crystal layer 133 shown in FIGS. 1 and 2 has the 3 o'clock reference orientation direction (vector c), provided FIG. 2 shows the liquid crystal panel in the usual direction for the viewers.
In TN LCD devices including liquid crystal molecules in the above-described orientation direction, the contrast of displayed images are different in accordance with the viewing angle. FIG. 3 is a graph illustrating the light transmittance of a liquid crystal panel of a TN LCD device as a function of the viewing angle. In FIG. 3, the horizontal line represents the viewing angle (.theta.v in FIG. 1), and the vertical line represents the light transmittance. A plurality of curves in FIG. 3 are obtained by different levels of the applied voltage. As is appreciated from FIG. 3, when the viewing angle is 0.degree. (perpendicular to the liquid crystal panel) or in the vicinity thereof, display of different tones between a white display to a black display can be realized by controlling the voltage. When the viewing angle of 10.degree. or more, the phenomenon referred to as "inversion" in which the tones of the images are inverted occurs. When the viewing angle is negative, the minimum transmittance increases as the absolute value of the viewing angle increases, thereby drastically reducing the contrast.
FIG. 4 is a graph illustrating the influence of the viewing angle on the voltage vs. transmittance curve (hereinafter, referred to as the "V-T" curve) in an LCD device of the NW mode. Curve L1 is obtained when the viewing angle is 0.degree.. As the viewing angle is shifted positive, the V-T curve is shifted left (curve L2). When the applied voltage exceeds a certain level, the transmittance increases. In other words, the contrast of displayed images is inverted at a certain viewing angle. Such a phenomenon occurs because the apparent birefringence changes in accordance with the viewing angle. In the case where the viewing angle is fixed, such a phenomenon occurs when the applied voltage changes.
The above-described phenomenon will be explained with reference to FIGS. 5A through 5C. FIGS. 5A through 5C schematically show that the apparent birefringence of the liquid crystal panel seen from a positive viewing angle changes in accordance with the applied voltage. When the applied voltage is 0 V or relatively low, as is shown in FIG. 5A, the liquid crystal molecule 133a in the central area of the liquid crystal panel seems to be elliptical for a viewer at position 37 looking at the liquid crystal panel with a positive viewing angle. Namely, the apparent birefringence .DELTA.n&gt;0. As the applied voltage is gradually increased, the liquid crystal molecule 133a in the central area is tilted toward the direction of the electric field. Thus, as is shown in FIG. 5B, the liquid crystal molecule 133a seems to be circular for the viewer at position 37. At this point, the apparent birefringence .DELTA.n=0, and thus the light transmittance is raised. As the applied voltage is further increased, the liquid crystal molecule 133a in the central area becomes almost parallel to the direction of the electric field. Thus, the liquid crystal molecule 133a seems to be elliptical again for the viewer at position 37. Namely, the apparent birefringence .DELTA.n&gt;0. Since the apparent birefringence (.DELTA.n) changes in accordance with the tilt angle of the liquid crystal molecule 133a in this manner, inversion occurs at a certain viewing angle.
Returning to FIG. 4, when the viewing angle is negative, inversion does not occur. However, when the absolute value of the negative viewing angle increases, the V-T curve is shifted right and is slow as is indicated by curve L3; that is, the contrast is drastically reduced.
Proposals for solving the problems of inversion and the reduction in contrast have been made in, for example, Japanese Laid-Open Patent Publication Nos. 60-211425 and 60-147722.
In Japanese Laid-Open Patent Publication No. 60-211425, the alignment layers are rubbed in different directions for different pixels to provide different T-V curves for the different pixels. Since the liquid crystal panel obtained in this manner has a plurality of different T-V characteristics, inversion in the positive viewing direction and the reduction in contrast in the negative viewing angle are alleviated.
In Japanese Laid-Open Patent Publication No. 60-147722, the alignment layers are rubbed in an arc to generate various reference orientation directions in a liquid crystal panel in an attempt of avoiding inversion and the reduction in contrast.
By the above-mentioned two methods, the problems are not sufficiently solved.
Japanese Laid-Open Patent Publication No. 5-107544 discloses still another technology for solving the dependence of the display characteristics on the viewing angle. In this reference, as is shown in FIG. 6, a plurality of liquid crystal regions having different reference orientation directions (indicated by the arrows c) are formed in each of a plurality of pixels. Each pixel is divided into, for example, two or four rectangular regions having the same surface area, and the reference orientation directions of adjacent regions are opposite to each other. In the case of a TN mode, the twist angle is set to be 90.degree..
FIG. 7 illustrates the light transmittance as a function of the viewing angle obtained in the LCD device disclosed in Japanese Laid-Open Patent Publication No. 5-107544. As is appreciated from FIG. 7, inversion in the positive viewing direction is alleviated, but the contrast is drastically reduced as the absolute value of the viewing angle is increased.
The technologies similar to the technology disclosed in Japanese Laid-Open Patent Publication No. 5-107544 are described in K. Takatori et al., Proceedings of Japan Display '92, pp. 591-594 and K. Kamada et al., Proceedings of Japan Display '92, p. 886. According to the technology described in pp. 591-594 of the above-mentioned publication, after the alignment layer is rubbed in one direction, a part of the alignment layer is covered with a resist and then rubbed in an opposite direction, thereafter removing the resist. As a result, the liquid crystal molecules corresponding to the part covered by the resist have a different reference orientation direction from that of the liquid crystal molecules in the part not covered by the resist. Such a method is referred to as the "twice rubbing method".
According to the technology described on p. 886 of the above-mentioned publication, alignment layers formed of different polyimide materials are located side by side and rubbed in the same direction. As a result, the liquid crystal molecules in positional correspondence with and in the vicinity of the respective alignment layers are tilted at different pretilt angles. Such a method is referred to as the "alignment layer patterning method".
In Japanese Laid-Open Patent Publication No. 60-211424, Complementary TN (CTN)--TN with a wider range of viewing angles--, Technical Report of The Institute of Electronics, Information and Communication Engineers (Japan), EID92-112, ED92-145, pp. 35-41, February 1993, and Japanese Laid-Open Patent Publication No. 5-188374, a method for dividing a pixel into a plurality of regions having different reference orientation directions and the like are disclosed.
These methods have the same problems as the method disclosed in Japanese Laid-Open Patent Publication No. 5-107544.
In Japanese Laid-Open Patent Publication No. 3-230120, a method using a compensation plate in order to partially change the viewing angle for the purpose of improving the contrast obtained at a certain viewing angle is disclosed. By this method, the viewing angle cannot be expanded in both the positive and the negative sides.